In the hydrocarbons (oil and gas) recovery industry, pipes can be connected by two threaded male and female unions which are designed and manufactured in accordance with the specifications defined in the American Petroleum Institute (API). This type of joint is also referred to as a “pin” and a “box” connection. The helical threaded joint engaging the two pipe sections defines a helical path through which fluid from the pipe section may leak.
Leakage is not acceptable in most situations for economical, environmental and safety reasons. In order to seal this helical path of potential leakage, pipe dope has been commonly used to coat the threads, a practice that is well known in the industry.
However, pipe dope alone can be inadequate to achieve fluid-tight sealing and some type of secondary O-ring seal is normally required. The secondary O-ring seal is usually placed in an annular recess in one of the box sections. When the pin and box are made up, the secondary seal is deformed between corresponding surfaces of the pin/box joint to achieve a good seal. The secondary seal ring can be made of polymeric or metallic materials.
Another type of metal-to-metal seal is the so called premium connection which employs tapered threads contoured in such a way that the mating threads always form a stressed metal-to-metal, circumferentially continuous, seal. Although this type of connection is better than other types, it requires a relatively high tolerance in machining and is more expensive.
Premium and conventional threaded connections have larger diameter than the body of a pipe (such as a casing used in oil well drilling operations), and hence a larger diameter hole must be drilled to run e.g. casings with threaded pipes. Larger diameter wells are slower to drill and therefore more expensive. Threaded connections are in general not as strong as the steel casing, so a threaded connection cannot withstand the same mechanical stresses as a casing (pipe) itself.
The hydrocarbon recovery industry has been experiencing steady increases in the cost of production due to having to retrieve hydrocarbons from deeper wells, harder rock formations and harsher environments. Deeper wells require more casing strings (pipe sections lining the hole), and therefore the hole diameter drilled is larger with threaded connections than if flush connections are used. Also, more complex geology often means harder drilling environments, and increased stresses on the casing whilst running in the drill hole. Threaded connections are often the weakest part of a drill string, and can prevent casings being rotated whilst running in hole due to the limited ability to withstand torque stresses of the threaded connection.
New technologies, such as expandable tubulars have been developed to reduce the loss of tubing diameter with depth that occurs in conventional drilling procedures. The technology involves forcing a tool down pipe sections to expand the diameter and thus allow more flow.
However, neither threaded nor premium connections work well for this type of application since they may lose sealing integrity, or even fail during the expansion operation.
On the other hand, traditional liquid welding techniques are also problematic as they may create weak spots/sections that could fail during the expansion operation due to inhomogeneous microstructures in the welds produced. In order to overcome this problem, forge welding has been proposed. Since forge welding is a solid joining process, it has the potential to generate more uniform microstructures in a weld thus is more suitable for expandable tubular technology.
Forge welding is a solid-state welding process that joins metallic structures by first heating the two faying sections to a high temperature, typically 50-90% of the melting temperature, then by the application of a forge force, followed by a controlled cooling or post weld heat treatment. Since this technique has the potential to generate high quality flush welds having more uniform microstructure and properties, it has been proposed for welding steel tubulars for well casings as well as for offshore pipeline construction, and for joining coil tubing.
Application of this technique to the joining of API tubulars is described for example in U.S. Pat. Nos. 4,566,625, 5,721,413 by Moe and U.S. Pat. Nos. 7,181,821, 7,774,917 by Anderson et al, and US Patent Application Publication No. 2011/0168693 by Rudd et al.
A typical tube or pipe forge welding operation begins at first by end profiling and cleaning to minimize the rust on pipe ends. Next, the tubes are loaded into a chamber which is then evacuated and then back filled with an inert gas. After the chamber has achieved the required conditions (e.g., pre-defined oxygen and water vapour levels), the tube ends are heated to the desired temperature (e.g., >1200° C.) with the protection of a so called shielded active gas (SAG) used to avoid oxidation at the joint being made. A force is then applied to the softened tube ends, forcing them together, to achieve forge welding. Depending on the steel types, forge temperature, and heating uniformity, the microstructures generated by these forge welding procedures may neither be ideal nor uniform, hence a post weld heat treatment is carried out. This may consist of allowing the steel to cool naturally, or by controlling the rate at which the steel cools, or by cooling the steel very quickly and then reheating it to relax the microstructure.
In order to achieve high quality welds the tube ends should be clean and oxide free. In addition the atmosphere in the forging chamber should contain minimum oxygen and water vapour content. Use is made of an SAG such as hydrogen/nitrogen mixture a reducing atmosphere with the intent of preventing new iron oxide formation during welding, but also to reduce residual iron oxide which are often present, even after careful cleaning procedures.
In practice, it usually is very difficult to achieve an oxygen and moisture free atmosphere and metal oxide free tube ends within a reasonable time span. Thus, a high quality forge weld cannot be guaranteed for every weld.
Moreover whilst pipes made of carbon steels, which have iron oxide and/or hydroxide as the predominant contaminant can be joined successfully with these prior art (SAG forge welding) techniques other steel grades, those containing more stable metal oxides such as chromium oxide are more problematic. It is more difficult to remove such oxides by reduction with e.g. hydrogen unless higher temperatures and longer times are used.
Thus there is a need for further improvements in forge welding techniques.